QHash value result when key is not in hash

Publicnamer

Looking at the documentation for QHash, it says that for some reason it will
construct a default object when the key doesn't correspond to an item in the hash.
But... what if I really just want it to return nullptr?
Do I need to use a different container for that behavior?

jsulm

@Publicnamer Do you mean operator[]?
Simply use https://doc.qt.io/qt-5/qhash.html#value-1

Publicnamer

@jsulm
Notice that it says:

If the hash contains no item with the key, the function inserts a default-constructed value into the hash with the key, and returns a reference to it. If the hash contains multiple items with the key, this function returns a reference to the most recently inserted value.

I want it to return nullptr when the item with my key isn't in the hash.

SGaist

Hi,

That's the single argument version documentation.

@jsulm suggests the overload that takes a second default value argument.

jsulm

@Publicnamer Please spend a bit more time to read documentation properly...

JonB

@Publicnamer
Although what @jsulm and @SGaist say about drawing your attention to const T QHash::value(const Key &key, const T &defaultValue) const is correct, it seems to me there is something wrong in your question.

If you want to "return nullptr" that must mean your value type is a pointer, e.g. QHash<QString, Something*>, not QHash<QString, Something>. And if that is so the default constructed value (for const T QHash::value(const Key &key) const) should already be nullptr. If your hash is in fact QHash<QString, Something> you won't be able to return nullptr, it is not type-compatible.

Is that not your case? If I am incorrect here I should be obliged if @jsulm or @SGaist corrects me.

P.S.
Be sure you use one of these QHash::value() methods, not the [] operator. The former return a value when not found but do not insert a new key-value into the hash, while the latter actually inserts a new key-value into the hash.

jsulm

@Publicnamer said in QHash value result when key is not in hash:

If the hash contains no item with the key, the function inserts a default-constructed value into the hash with the key, and returns a reference to it. If the hash contains multiple items with the key, this function returns a reference to the most recently inserted value.

This is from https://doc.qt.io/qt-5/qhash.html#operator-5b-5d NOT from https://doc.qt.io/qt-5/qhash.html#value-1 !